HIVE内置函数map_keys -- 源码解析

HIVE内置函数map_keys

所在的全类名

org.apache.hadoop.hive.ql.udf.generic.GenericUDFMapKeys
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一、initialize方法

@Override
  public ObjectInspector initialize(ObjectInspector[] arguments)
  throws UDFArgumentException {
    // 传入的参数为1个
    if (arguments.length != 1) {
      throw new UDFArgumentLengthException("The function MAP_KEYS only accepts one argument.");
    } else if (!(arguments[0] instanceof MapObjectInspector)) {
      // 参数类型应该是MAP类型
      throw new UDFArgumentTypeException(0, "\""
          + Category.MAP.toString().toLowerCase()
          + "\" is expected at function MAP_KEYS, " + "but \""
          + arguments[0].getTypeName() + "\" is found");
    }
		// 参数强转为MapObjectInspector
    mapOI = (MapObjectInspector) arguments[0];
    // 返回一个map的key检查器
    ObjectInspector mapKeyOI = mapOI.getMapKeyObjectInspector();
    // 返回一个列表
    return ObjectInspectorFactory.getStandardListObjectInspector(mapKeyOI);
  }
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在上面的源码之后，需要知道返回的列表是怎么执行的，点进去ObjectInspectorFactory类看getStandardListObjectInspector方法

public static StandardListObjectInspector getStandardListObjectInspector(ObjectInspector listElementObjectInspector) {
    StandardListObjectInspector result = (StandardListObjectInspector)cachedStandardListObjectInspector.get(listElementObjectInspector);
    if (result == null) {
        result = new StandardListObjectInspector(listElementObjectInspector);
        StandardListObjectInspector prev = (StandardListObjectInspector)cachedStandardListObjectInspector.putIfAbsent(listElementObjectInspector, result);
        if (prev != null) {
            result = prev;
        }
    }

    return result;
}
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其中cachedStandardListObjectInspector的声明如下

static ConcurrentHashMap<ObjectInspector, StandardListObjectInspector> cachedStandardListObjectInspector = new ConcurrentHashMap();
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二、evaluate方法

@Override
  public Object evaluate(DeferredObject[] arguments) throws HiveException {
    retArray.clear();
    Object mapObj = arguments[0].get();
    Map<?,?> mapVal = mapOI.getMap(mapObj);
    if (mapVal != null) {
      retArray.addAll(mapVal.keySet());
    }
    return retArray;
  }
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getDisplayString方法

@Override
  public String getDisplayString(String[] children) {
    assert children.length == 1;
    return getStandardDisplayString("map_keys", children);
  }
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getStandardDisplayString的方法：

protected String getStandardDisplayString(String name, String[] children) {
    return getStandardDisplayString(name, children, ", ");
  }
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调用了重载方法，一点点的把数组内的数据append到字符中

protected String getStandardDisplayString(String name, String[] children, String delim) {
  StringBuilder sb = new StringBuilder();
  sb.append(name);
  sb.append("(");
  if (children.length > 0) {
    sb.append(children[0]);
    for (int i = 1; i < children.length; i++) {
      sb.append(delim);
      sb.append(children[i]);
    }
  }
  sb.append(")");
  return sb.toString();
}
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